Compared to conventional fibrous or foam insulation, the superior heat-keeping or cold-keeping characteristics of a vacuum-insulated Thermos bottle are well known. However, large vacuum-insulated panels are not now commonly used because available designs do not perform as well as a small vacuum bottle, and equivalent poor performance can be achieved with foam or fibrous materials at lower cost.
Factors that favor vacuum insulation are:
1. A good vacuum substantially eliminates heat losses due to conduction, convection, and infiltration. PA0 2. Vacuum-insulated panels can be substantially thinner than panels insulated to an equivalent degree with foam or fibrous material. PA0 3. Moisture cannot penetrate a vacuum insulation and destroy it with ice accumulation, as it can with low temperature applications of foam or fiber insulation. PA0 1. Large thin-wall panels require internal spacers to resist the compressive force of the outside atmosphere and to hold the sides apart. These spacers, which must support over one ton per square foot of panel, have been significant paths for heat flow by conduction through the panel. PA0 2. Heat flow by radiation can be significant through a vacuum, particularly at higher temperatures or when the inside surfaces of the hollow panel are not mirror-bright. PA0 3. It is not easy to achieve and to maintain the low pressures reportedly used in vacuum bottles, around 10.sup.-3 Torr (approximately one millionth of normal atmospheric pressure). The edge seals must be designed to accommodate the thermal expansion of the warm side of the panel relative to the cooler side. Gases dissolved in metals and moisture absorbed on glass and metal surfaces can be released slowly to impair a high vacuum unless heat is applied during the evacuation or long evacuation periods are used. Most rubber and plastic materials also contain gases or they allow gases to diffuse through them. PA0 A. Conduction: Heat flows through a stationary material. PA0 B. Convection: A recirculating fluid carries heat from a warmer surface to a cooler surface, or from a warmer fluid to a cooler fluid. PA0 C. Radiation: A warmer surface emits electromagnetic type waves to a cooler surface in visual sight or to outer space. PA0 D. Infiltration or exfiltration: A warmer fluid passes through a partition and is replaced by a cooler fluid.
On the other hand, some disadvantages of large vacuum-insulated panels are:
A general review of heat transfer mechanisms may be helpful toward understanding this invention. Heat may flow through a panel or partition by any combination of the following mechanisms:
Different materials have different tendencies to conduct heat. These tendencies can be measured and reported as "the coefficient of thermal conductivity." In British units, the standard path for heat flow is one square foot in cross section area, and the path is one inch long. The amount of heat per unit of time, or B.T.U. per hour, transferred per degree of temperature difference is reported as the coefficient. Such coefficients may vary with both the temperature and the purity of material tested. Some reported coefficients are shown below.
______________________________________ Coefficient of Thermal Conductivity Material (B.T.U./hour ft.sup.2 .degree.F./inch) ______________________________________ Copper 2600 Aluminum 1400 Steel 320 Stainless Steel 110 Glass 7 Concrete 3 Polyester Plastic 1.6 Polystyrene foam 0.24 Still Air 0.16 (Vacuum) 0.000 ______________________________________
It is obvious that thicker walls conduct proportionately less heat. For walls that are made of two or more materials, or are hollow, or insulated, it is more convenient to measure an overall "U Factor." Some reported values are:
______________________________________ U Factor Panel or Wall Construction (B.T.U./hr ft.sup.2 .degree.F.) ______________________________________ 8" concrete blocks, faced with brick; air space between 0.4 as above, with additional gypsum wall board and air space 0.18 Sheet steel on both sides of 11/2 inch fiberglass 0.14 ______________________________________
Heat transfer by radiation is normally relatively small compared to heat transfer by the other mechanisms at ambient temperatures or less. Radiation losses increase with the fourth power of the absolute temperature, .degree.R., as follows:
______________________________________ .degree.F. .degree.R (R/100).sup.4 Ratio ______________________________________ -30 430 342 0.43 0 460 448 0.57 70 530 789 1.00 140 600 1296 1.64 240 700 2401 3.04 340 800 4096 5.19 ______________________________________
Radiant energy (infra-red radiation at ambient temperatures) readily passes through a vacuum. At the next surface, this energy is either absorbed, reflected, transmitted, or some combination of these. In an ideal vacuum insulation system, the internal surface would reflect all the radiant energy.
It is not practical to make a perfect vacuum or a perfect reflector. Measurements of the U Factor of a common Thermos bottle showed the following:
______________________________________ Equivalent thickness Temperature Range U Factor Polystyrene foam ______________________________________ Boiling water to room temperature 0.071 3.38 inches Ice water to room temperature 0.036 6.67 inches ______________________________________
Data on the degree of vacuum and the percent reflectivity of the inside surfaces were not available.